Radio bomb release system



' Nov, 1e, 194s.

Filed July 24, 1944 J. `H- LUDWIG RADIO BOMB RELEASE SYSTEM 2 Sheets-Sheet l I? T 706%!!! Y Nov. 1948. y J, H, LUDwlG 2,453,996 l RADIO BOMB RELEASE SYSTEM l Filed July 24. 1944 2 Sheets-Sinaai 2 Mowmvm/ M01/amd T5 @wi-+75, anw sul??? INVENTog.

TW/VEY Patented Nov. 16, 194s 2,453,996l RADIO BOMB RELEASE SYSTEM James H. Ludwig, Collingswood, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application July 24, 1944, Serial No. 546,269

This invention relates to radio bomb release systems, and more particularly to improvements in systems of the type described in copending ap-v plication Ser. No. 524,794, filed on March 2, 1944, now Patent No. 2,412,632, dated December 17, 1946, by Royden C. Sanders, Jr., and William R. Mercer, and entitled Radio bomb release system, wherein a frequency modulated signal is radiated from a mobile craft toward a selected target, received after reflection from said target, and the received signal compared with the transmitted signal to actuate a bomb release device upon the occurrence of a predetermined relationship'between the target distance and the speed with respect to the target. l

Systems of the described type'are sometimes l subject to disturbances caused by the absence of a desired target or caused by severe fading of the received signals, as a result of multiple path transmission between the target and the bombing craft. Another effect which is almost invariably present in the operation of such systems is that of a sea return, or reflection from the surface upon which the target lies. These two eects will, on occasion, conspire to cause premature release during pronounced fading conditions, since the so-called sea return will over-ride the target signal, simulating a signal from a target at minimum range. A similar effect may also be produced by improper adjustment of the radio system.

It is the principal object of the present invention to provide a method of and means for preventing such premature releases. Another object is to provide a method of and means for causing the radio equipment to operate during fading conditions as if the target were at maximum range rather than minimum range, thus preventing release when no signal is being received from the target.

These and other objects will become apparent to those skilled in the art upon consideration of the following description with reference to the accompanying drawings of which Figure 1 is a schematic diagram of a bomb release system embodying the invention,

Figure 2 is a graph illustrating variations in frequency of signals transmitted and received in the operation of the system of Figure 1,

Figures 3 and 4 are graphs of square wave switching voltages occurring in the operation of Figure 1, and

Figure is a graph illustrating the switched counter output currents produced in the operation of the system of Figure 1A.

4 Claims. (Cl. 343-7) 2 .Refer toFigure 1. A system of the type described in the above-mentioned Sanders et al. ap-

plication is illustrated. A radio transmitter I is v voltage,

connected to an antenna 3 and to a frequency modulator 5. The modulator 5 may be of the vibratory variable capacitor type such as that described in copending U. S. Patent application Serial No. 471,003, filed January 1, 1943, by S. V. Perry and entitled Capacity modulator unit, or any other known device for varying the frequency of `the transmitter l in response to a modulating The input circuit of the modulator 5 is connected to a wave shaping circuit 'I which is connected through a voltage divider 9 to a battery II and a periodic switch I3. The switch I3 is ar ranged to be operated by a cam I5 driven by a motor I1. The motor I'I is connected through a switch I9 to a power source such as a battery 2i.

A receiving antenna, 23, similar in construction to the antenna 3, is connected to a detector 25. The transmitter I is also connected to the detector 25 through a line 24. Both antennas 3 and 23 are preferably directive, and are arranged to provide maximum response in the same direction.

The output circuit of the detector 25 is conlnected to an amplifier 29, which is provided with an A. V. C. circuit including a rectifier 30 connected to rectify a portion of the amplifier output, and a filter comprising capacitors 26 and 2l and a resistor 28, through which the rectified voltage is applied to a bias circuit of the amplier in any known or conventional manner for controlling the amplifier gain. The output of the rectifier 30 is also applied to the control grid of an electron discharge tube 32. A D. C. source comprising a battery 36 and a voltage divider 39 is connected to the rectifier 30 and adjusted to bias it so that no rectication will occur with less than a predetermined output from theamplier 29. The output circuit of the amplifier 29 is connected to an amplitude limiter 3i. The Output circuit of the limiter 3l is connected to a pair of averaging cycle counter circuits, generally designated by the reference numerals 33 and 35, respectively.

The counters 33 and 35 are provided with a common load resistor 31, which is connected to the control grid of an amplier tube 39. The anode of the tube 39 is connected directly to the D.C. source 36, The cathode circuit of the tube 39 includes a resistor 43' tapped at a point 43. The catho-de of the tube 39 is connected to the cathode of a tube 46. The control grid of the tube 46 is connectedto a bias source comprising a voltage divider '50, connected across the battery 36, A relay 35 is included in the anode circuit ofthe tube 45.

The counter 33 includes a capacitor 49 connected to the cathode of a diode 5i and to the anode of a triode 53, The control grid of the triode 53 is coupled to the switch I3. The cathode of the tube 53 is connected to the cathode of the tube 39. The anode of the diode 5I is connected to the load resistor 31, The counter comprises a capacitor 55 connected to the cathode of a diode 51 and to the anode of a trode 59. The control grid of the triode 59 is coupled through a phase inverter circuit 6I to the switch I3. The anode of the diode 51 is connected to the cathode circuit of the tube 39 at the point 43. The cathode of the triode 59 is connected to the control grid of the tube 39 and the upper end of the resistor 31, The lower end of the resistor 31 is connected to a voltage divider 63 across the battery 36. The load resistor 31 is bypassed to ground by a capacitor 61.

The operation of the system thus far described is as follows: The motor I1 operates the switch ing circuit l. The wave shaping' circuit 1 includes a low pass filter or other means for integrating the square wave input with respect to time to produce an output of triangular wave shape. The triangular wave output of the wave shaping circuit 1 is applied to the modulator 5, causing corresponding triangular wave variation of the frequcncy of operation of the transmitter I. The frequency modulated signal produced by thc transmitter I is radiated by the antenna 3 to the target, not shown, Part of the energy striking the target is reflected to the receiving antenna 23.

The received reflected signal is combined in the detector 25 with some of the original frequency modulated signal, which is conducted directly from the transmitter I-to the detector 25 through the line 23, The output of the detector 25 includes a beat signal having a frequency equal to the difference between the instantaneous frequency of the transmitted and received signals. The beat output of the detector 25 is amplified by the amplifier 29 and limited to a constant amplitude by the limiter 3|. The output of the limiter 3| is a square wave Voltage having a frequency equal to the difference between the frequency of the transmitted and received signals and a constant amplitude, Es.

Referring to Figure 2, the frequency of the transmitted signal is represented by the solid line 13. This frequency varies throughout the modulation cycle in response to the triangular wave output of the wave shaping circuit 1, between *upperl and lower limits f2 and f1, respectively, about a mean value fo. The sweep width fz-fi is proportional to the amplitude of the triangular wave input to the modulator 5, and hence is a function of the position of the adjustable contact of the voltage divider 9.

The reflected signal is delayed with respect t0 the transmitted signal by the time required for the radiation to travel from the transmitting antenna 3 to the target, and back to the receiving antenna 23, This is indicated by the dotted line 'l5 in Figure 2. The reflected signal varies in lil frequency over the same range fz-fi as the transmitted signal, but constantly differs in frequency from the transmitted signal by an amount proportlonal to the distance. This difference in frequency is cycles per second, where S=fzf1 in megacycles per second, fm is the modulation frequency in cycles per second, or frequency of operation of the switch I3, and d is the distance in feet. If the equipment is moving toward the target, the received signal is increased in frequency, owing to Doppler effect, by an amount gl'fn where v is the velocity in feet per second, fo is the carrier frequency in cycles per second and C is the velocity of radiation propagation in feet per second. The frequency of the received signal is represented by the dash line 11 in Figure 2. This frequency variesthroughout the modulation cycle over a frequency band which is equal in width to that of the transmitted signal. The difference in frequency between the transmitted and the received signal is cycles per second during increase in frequency of the transmitted signal, and

Sfmd afi fd" 246 C cycles per second during decrease in frequency of the transmitted signal.

The constant amplitude beat frequency output of the limiter 3| is applied to both of the counters 33 and 35. The connections to the modulator 5 are such that during the modulation downsweep. or decrease in frequency of the transmitted signal, the switch I3 is closed, applying a positive pulse to the control grid of the triode 53 and to the phase inverter 6I, as indicated by the graph 'of Figure ie/The phase inverter 6I provides a negative pulse which is applied to the control grid of the triode 59 of the counter 35, as indicated by the graph of Figure 3. The triode 59 is thereby cut oi, and the counter 35 prevented from operating.

During negative half cycles of the limiter output, the capacitor .49 is charged through the diode 5I and the resistor 31. The values of the capacitor 49 and resistor 31 are such that the capacitor 39 becomes substantially fully charged to the limiter output voltage Es during each cycle of the limiter output. During positive half cycles, the capacitor 49 is discharged through the triode 53 to the potential existing at the cathode of the tube 39, which is substantially equal to that at the anode of the diode 5 I. Thus during the modulation downsweep, the counter 33 causes an average current id to ow upward through the resistor 31 as indicated by the dash arrow. This current is proportional to the product of the charge deposited in the capacitor 49 during each cycle, and the number of cyclesper second:

where C2 is the capacitance of the capacitor 49. Since During the modulation upsweep, the switch I3 is open, providing a negative pulse at the grid now inoperative and the counter 35 operates.

The triode 59 is conductive, allowing the capacitor 55 to charge through the resistor 31 during positive half cycles of the output of the limiter 3|. During negative half cycles of the limiter output, the capacitor 55 is discharged through the diode 51 to the potential appearing at the tap B4 on'the resistor d3, which is slightly less than the potential at the cathode of the tube 39 and hence that of the cathode of the tube 53. Thus during the modulation upsweep, the counter 35 causes a current iu to flow downwardthrcugh the resistor 31, as indicated by the solid arrow. This current-is proportional to the product of the charge deposited in the capacitor 55 during each cycle, and the number of cycles per second:

u :fuQ :flClEl r where fu is the beat frequency. Q is the charge per cycle, C1 is the capacitance of the capacitor 55, and E. is the amplitude of the output of the limiter 3|. Since l dia-ghn sigma c.) fiaba (01+ c.)

Refer to Figure 5, whereinV I1 is the average component of current during upsweep due to distance, I2 is the average component oi current during downsweep due to distance, Iz is the resultant-average component of current due to distance, I4 is the increase in negative average component of current during downsweep due to speed, and I5 is the decrease in positive average current during upsweep due to speed. The resultant average voltage across the resistor 31 is en=ioR, where R is the resistance of the resistor 31.

The tap of the voltage divider 63 is adjusted to apply a positivepotential of, for example, approximately 70 volts to the lower end of the resistor 31. The purpose of this arrangement is to provide a suitable operating point for the cathode follower tube 39. Denoting this voltage as er, the total voltage at the control grid of the tube 39, referred to ground'potential, is eo+e1. Inasmuch as the entire load of the tube 39 is in the cathode circuit, the anode current will assume a value suchl that the drop in said load circuit is very slightly greater than the voltage between the control grid and ground, 'and as a practical matter, substantially equal to the grid voltage.

This is the case only so long as the tube 46 is nonconductive. The voltage divider 50 is adjusted to apply a positive voltage e2 to the control grid of the tube 46. Since the cathode is also Ipositive, at a voltage substantially equal to eo|e1, the tube 46 will be cut off as long as eo-l-ei exceeds e2 by more than a predetermined voltage which depends upon the design of the tube 46 and the anode voltage supplied thereto by the battery 33.

the upsweep and downsweep oi the modulation The voltage ez is adjusted in accordance with the altitude at which a bombing run is to be made. as described in detail in the abovementioned Sanders et al. application.

Assume that the bombing craft is travelling toward the target at a, substantially constant velocity v. Initially, the distance d is relatively great, and hence io is relatively large. The voltage eo is therefore of maximum value, and eu-i-ei greatly exceeds ez. As the target is approached, e0 gradually decreases, while e1 and e2 remain constant. At a predetermined time T before the bomber is directly over the target, eo falls to a value low enough so that the tube 46 will conduct, energizing the relay 45 to actuate the bomb release mechanism. The voltage dividers 9, 5U, and 63 are adjusted as described in the above-mentoned Sanders et al. application to make the time T equal to that of that required for a bomb to fall from the altitude h at which the bombing run is to be made.

Under ideal conditions, the above-described operation Will cause release to occur at the proper instant to score a hit upon the selected target. Ordinarily the signal reflected by the target is stronger than that reflected by the surface upon which the target is lying, so that the voltage eo is a function of substantially only the target distance and the speed of the bomber with respect to that ofthe target. However, if the target signal fades, surface reflections will be picked up by the apparatus. These reflections may simulate a signal froma target directly under the bomber or that from a target at a distance which is much less than the distance of the actual target. Accordingly, the voltage eo will decrease, and may decrease to such an extent that the tube 46 is allowed to conduct, 'actuating the relay 45 and causing release of a'bomb."

This diiculty, although caused by the absence of thev target or by fading,'is .not remedied by the A. V. C. device, which merely tends to maintain the general signal level, with no discrimination between target reflection and surfacereflection.

In accordance with the instant invention, the tube 32 is provided with its an-ode connected to the grid circuits of the phase inverter 6I and the triode l53 of the negative counter 33. The cathode of the tube 32 is grounded, and the control grid is connected to the output circuit of the A. Vs'C. rectifier 30.

Under normal conditions of operation, as described above, relatively strong reilection signals are received from the target, overcoming the threshold bias derived from the voltage divider 38, so that the rectifier 30 operates. This provides a negative voltage to the gain control circuit of the amplifier 29 and also to the control grid of the tube V32. The voltage divider 38 is adjusted so that the tube 32 is cut oiwhen the rectifierv 30 operates, and therefore has no eiiect upon the voltages at the grids of the tubes 53 and 6|. When the received signal falls below a predetermined amplitude, the rectifier 39 ceases to operate and the voltage at the grid of the tube 32 becomes substantially zero. The tube 32 then presents a low resistance from anode to cathode, eiectively grounding the control grids of the tubes 53 and 6I. Since the cathodes of the tubes 53 and 6| are at a high positive potential with respect to ground, these tubes are cut off. The counter 33 causes to operate, and the counter 35 operates continuously during both attacca cycle. e current through the resistor 8l, consequent y, is substantially 2i rather than iu-ia. and the voltage applied to the control grid of r.the tube 39 is'highly positive, corresponding to the voltage e which would appear as a result of reection from a target at a great distance. Thus so long as the received signal is lower than a predetermined amplitude, the tube it is prevented from conducting.

If the target signal returns before the bomber reaches the target, the rectifier 30 starts to operate again, cutting of! the tube'82 and allowing the differential counters 33 and 35 to operate in normal fashion, causing a normal release t\ occur.

Thus the invention has been described as an improved radio bomb release system, including means for preventing false release upon failure of the target Vreflection signal by simulating response to a target at a great distance. In the described system, this is accomplished by means of a single tube connected to respond to the automatic volume control rectier to disable one of the diierential counter circuits and cause the other to operate continuously.

I claim as my invention:

1. A system for preventing false operation of a radio bomb release system oi the type including a frequency modulated transmitter, a receiver, and alternately operable differential counters connected to said receiver, including normally inoperative means for preventing operation of one of said counters, and means responsive to the amplitude of signals received by said receiver to operate said normally inoperative means upon decrease of said amplitude below a predetermined value.

2. In a radio bomb release system of the frequency modulation reflection type including a transmitter and a receiver and two alternately operable beat frequency responsive circuits connected to Saidreceiver and providing opposed outputs to effect release upon the occurrence of a predetermined difference in magnitude between said outputs, means for preventing false operation of said system comprising an automatic gain control circuit connected to said receiver and responsive to the amplitude of the input thereto to control the gain of said receiver, and means responsive to the output of said automatic gain control circuit to prevent operation of one of said frequency responsive circuits continuously during periods when the amplitude of said input is less than a predetermined value.v

3. In a radio system including a frequency modulated transmitter, a receiver, differential counter circuits connected to said receiver and switching means for operating said-counters respectively during increase and decrease of frequency of the output of said transmitter. a system for preventing false operation of said system during signal fading conditions comprising means responsive to the amplitude of signal received by said receiver to control the sensitivity of said receiver, an electron discharge tube connected to prevent operation of said switching means, and means for biassing said tube in response to said sensitivity control means to be normally inoperative, whereby decrease oi said received signal below a predetermined magnitude prevents operation of one of said counters and causes continuous operation of the other.

4. A system for preventing false operation oi' a radio bomb release system of the type including a radio transmitter, a source of modulating signal, means responsive to said modulating signal to vary cyclically the frequency of the output of said transmitter, means for radiating said output to a selected target, means for receiving said signal after reflection` by said target and for combining said received signal with said transmitter output to produce `a beat signal, said receiving means including an automatic gain control system, two differentially operable beat frequency responsive circuits coupled to said receiving means and provided with a common load circuit, and means including an input circuit coupled to said modulating signal source and responsive thereto to disable one of said frequency responsive circuits during increase of frequency of said transmitted signal and to disable the other of said frequency responsive circuits during decrease of frequency of said transmitted signal; said subject system comprising an electron discharge tube including at least an anode, a cathode, and a control grid, connections from said anode and said cathode to respective terminals of said modulating signal input circuit of said disabling means, whereby said tube, when in conductive condition, effectively short circuits said input circuit, and means for applying the output of said automatic gain control circuit to said control grid to bias said tube to substantially cut-ofi' condition in the presence of a received signal of greater than a predetermined amplitude, whereby one of said frequency responsive circuits operates continuously upon failure of said reflected signal, producing a voltage across said common load circuit of a magnitude corresponding to that which would appear in normal operation in response to signals reflected from a substantially remote target.

JAMES H. LUDWIG.

No references cited. 

